Process for making pressure-sensitive adhesive and duct tape

ABSTRACT

A continuous process for making a pressure-sensitive adhesive is disclosed. A mixture comprising natural rubber having a Mooney viscosity of 85 to 100, a tackifier, a filler, and 0.1 to 5 wt. % of an added C 12 -C 24  fatty acid based on the amount of mixture is masticated in a first section of a single- or twin-screw extruder. Mastication of the mixture continues in at least one subsequent extruder section in the presence of additional tackifier. The product is a homogeneous, reduced-viscosity pressure-sensitive adhesive. The minor proportion of added C 12 -C 24  fatty acid aids mastication of the rubber and enables high throughput without addition of peptizers. Duct tapes made from the adhesives display improved adhesion to steel, better adhesion bond strength, and enhanced seven-day clean removability from even difficult substrates such as marble or ceramic tile.

FIELD OF THE INVENTION

The invention relates to a process for making natural rubber-basedpressure-sensitive adhesive compositions and to duct tapes that utilizethe adhesives.

BACKGROUND OF THE INVENTION

Pressure-sensitive adhesives (PSA) for duct tape applications arenormally based on natural rubber and/or synthetic elastomers such aspolyisoprene. Because untreated natural rubber has a molecular weightgreater than about 400,000, it needs to be broken down to more usefulmolecular weights (e.g., about 100,000) for use in pressure-sensitiveadhesives.

The high molecular weight of natural rubber is usually reduced bymastication in a batch process using a Banbury or similar mixer. Atypical formulation includes natural rubber, filler, tackifier, andoptionally a process oil. “Constant viscosity” grades of rubber, such as“CV60,” which has a Mooney viscosity of 60, are available. However,because of the extra processing steps needed, these CV grades arerelatively expensive. “Technically specified rubber” (or “TSR” graderubber) has higher molecular weight and is less expensive. TSR grades ofrubber are produced in a range of Mooney viscosities depending on theplantation and country of origin. Processing time required in a Banburymixer depends on the initial molecular weight of the rubber. In general,higher molecular weight rubbers require a longer mixing time, whichhampers productivity.

Extrusion processes for making pressure-sensitive adhesives are known.U.S. Pat. Nos. 5,539,033 and 6,166,110 disclose a continuous extrusionprocess for making a PSA. Single sources or blends of rubbers are taughtas suitable. The references teach to use natural rubbers such as CV60,ribbed smoked sheet, and synthetic rubbers such as polyisoprene andstyrene-butadiene rubber (SBR). U.S. Pat. No. 6,777,490 teaches to usean extruder to process an aqueous rubber latex mixture. Relatively hightemperatures (up to 170° C.) are needed to evaporate water from theformulation during adhesive processing. U.S. Pat. Nos. 6,506,447;6,780,271; and 7,476,416 teach to use a modified planetary rollerextruder to process rubber. The '447 and '271 patents refer to theprocess as “mastication-free,” while the '416 patent refers to“purposeful mastication” in the planetary roller extruder. According tothe '447 and '271 patents, suitable planetary roller extruders can have7-24 screws or spindles. In general, planetary roller extruders are farmore expensive to obtain and maintain when compared with single-screw ortwin-screw extruders.

Although low capital investment, high production rates, and lowprocessing costs have made the continuous extrusion process for makingPSAs more attractive in recent years, there are several importantdrawbacks.

We found, for instance, that continuous extrusion processes can providehomogeneous adhesives at high throughput rates when the natural rubberhas low to medium molecular weight (as with the CV60 natural rubberdescribed in the '033 and '110 patents). However, when higher molecularweight natural rubbers are used, longer residence times are needed inthe extruder to achieve adequate mastication of the rubber.Consequently, the high throughput rates that are a principal advantageof continuous extrusion are sacrificed.

Another option is to add a peptizer (e.g., zinc soaps of unsaturatedfatty acids or aromatic disulfides) into the rubber formulation tochemically assist in the breakdown of the high-molecular-weight rubberin the extruder without reducing throughput. However, residual peptizerin the PSA can disrupt aging properties of the resulting tape.

Natural rubber contains traces of various fatty acids, including stearicacid, oleic acid, linoleic acid, and others (see A. Arnold et al., “Roleof Fatty Acids in Autooxidation of Deproteinized Natural Rubber, J. Nat.Rubber Res. 6 (1991) 75). Fatty acids are sometimes included in rubbercompounding processes to enhance vulcanization, particularly inprocessing rubber for tire manufacture (see, e.g., GB 1,532,294 and U.S.Pat. No. 3,900,999), or to accelerate crosslinking by other activecomponents such as zinc oxide (see, e.g., U.S. Pat. No. 6,780,271).

Duct tapes are constructed using a flexible backing layer, e.g.,low-density polyethylene or the like, a cloth scrim, and a rubberadhesive that can penetrate openings in the scrim and bond the scrim tothe backing layer (see, e.g., U.S. Pat. Nos. 4,303,724; 4,992,331;5,108,815; 5,271,999). A challenge with duct tapes is in providing ahigh level of adhesion while also permitting clean removability of thetape after use. Duct tapes tend to leave behind considerable adhesiveresidue, particularly when the substrate to which the tape has beenattached is marble, ceramic tile, laminate flooring, or carpeting.

A need remains for new ways to manufacture pressure-sensitive adhesivesfor tape applications from high-molecular-weight natural rubber in acontinuous extrusion process without sacrificing productivity andwithout reliance on chemical peptizers or high process temperatures.Ideally, the expensive and complex planetary roller extruders could beavoided. The industry needs tapes that have strong adhesion on substratesurfaces when applied but leave no residue when the tape is subsequentlyremoved, especially from difficult substrates.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a two-step continuous processfor making a pressure-sensitive adhesive. A first step comprisesmasticating in a single- or twin-screw extruder a mixture comprisingnatural rubber, a tackifier, a filler, and an added C₁₂-C₂₄ fatty acid.The mastication is performed in a first section of the extruder at atemperature within the range of 10° C. to 45° C. The natural rubber hasa Mooney viscosity within the range of 85 to 100. An amount of fattyacid within the range of 0.1 to 5 wt. % based on the amount of mixtureis used. In a second step, mastication of the mixture continues in atleast one subsequent extruder section at a temperature within the rangeof 45° C. to 100° C. in the presence of additional tackifier. Theproduct is a homogeneous pressure-sensitive adhesive having a viscosity,measured at 150° C. and a shear rate of 5000 s⁻¹, at least 15% lowerthan the viscosity of a pressure-sensitive adhesive prepared by asimilar process without the added C₁₂-C₂₄ fatty acid.

In another aspect, the invention relates to a process for making ducttape. The process comprises preparing a pressure-sensitive adhesive asdescribed above. The adhesive is then calendered onto a backingcomprising a polyolefin and a cloth scrim to produce the duct tape.

We surprisingly found that pressure-sensitive adhesives produced byextrusion according to the inventive process in the presence of a minorproportion of an added C₁₂-C₂₄ fatty acid can be produced from evenhigh-molecular-weight natural rubber with short residence times andwithout the need for peptizers or a more expensive extruder. Given thatnumerous fatty acids occur naturally in natural rubber, it is remarkablethat introduction of a minor proportion of a C₁₂-C₂₄ fatty acid wouldprovide benefits in aiding mastication of rubber having high molecularweight. Duct tapes produced using the adhesives display improvedadhesion to steel, better adhesion bond strength, and enhanced seven-dayclean removability from even difficult substrates such as marble orceramic tile.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 represents a twin-screw extruder in accord with one aspect of theinvention. The extruder has six sections (S1-S6) for transferring and/ormasticating rubber mixtures and three hoppers (H1-H3) for introducingsolid adhesive components. In some aspects, there may be three liquidinjection ports (not shown) located near hoppers H1-H3 for introducingmolten resins, process oils, or other liquid components.

DETAILED DESCRIPTION OF THE INVENTION

The Single- or Twin-Screw Extruder

The process for making the pressure-sensitive adhesive is performed in asingle- or twin-screw extruder. The particular design of the extruder isnot believed to be critical. Thus, when a twin-screw extruder is used,it may be co-rotating or counter-rotating and the screws may or may notintermesh. Single- and twin-screw extruders are convenient, costeffective, and commercially available from numerous manufacturers orsuppliers. In preferred aspects, a twin-screw extruder is used. Suitableextruders are commercially available from Maris, Coperion, W.W.Brabender Instruments, Farrel Pomini, and others. One particularlysuitable example is a co-rotating twin-screw extruder supplied by F. IliMaris, S.p.A., Italy, having a screw diameter of 150 mm and a length todiameter ratio of 54.

The extruder will normally have multiple sections utilized forcombining, mixing, heating, cooling, equilibrating, conveying,masticating, or some combination of these tasks. Temperature can becontrolled to a desired value in each section. Usually, two or more ofthe sections will be equipped with a hopper for introducing solidcomponents or a liquid injection port for liquid components. A suitableextruder configuration is pictured in FIG. 1. In this case, thetwin-screw extruder has six sections (S1-S6) for transferring and/ormasticating rubber mixtures and three hoppers (H1-H3) for introducingadhesive components.

Usually, the temperature in the extruder sections increases to somemaximum value near the exit of extruder, although in some cases it maybe desirable to cool the product in one or more sections prior to itsexit. Some exemplary temperature ranges for the extruder configurationshown in FIG. 1 are as follows: S1: 70° F.; S2: 70° F.; S3: 100° F.; S4:115° F.; S5: 115° F.; and S6: 170° F.

A benefit of continuous extrusion is productivity. Desirably, theresidence time of material in the extruder will be less than 10 minutes,preferably less than 5 minutes, and in some cases less than 3 minutes.This compares favorably with batch mixing processes, which will normallyrequire 0.5 to 2 hours or more to process an equivalent amount ofpressure-sensitive adhesive.

By “continuous,” we mean that the process operates continuously as longas there is a need to supply pressure-sensitive adhesive for itsdownstream use. For instance, the process for generating thepressure-sensitive adhesive can proceed continuously while othercomponents for making the tape or other product are available and readyto utilize the continuously generated adhesive.

Natural Rubber

The inventive process uses natural rubber. Although certain syntheticrubbers might be used in combination, there is no apparent advantage indoing so. The inventive process makes it practical to use natural rubberhaving high molecular weights. In particular, natural rubber usefulherein has a Mooney viscosity within the range of 85 to 100, preferably90 to 98. This corresponds generally to natural rubber having aweight-average molecular weight (M_(w)) as measured by gel permeationchromatography of at least 0.5×10⁶ g/mol, preferably at least 1×10⁶g/mol, or 1.5×10⁶ g/mol, and preferably an M_(w) within the range of1.6×10⁶ g/mol to 1.0×10⁷ g/mol or 1.6×10⁶ g/mol to 5.0×10⁶ g/mol. Thesource of the natural rubber is not believed to be critical.

Suitable natural rubber will have a polydispersity of at least 4,preferably at least 4.5, or in some aspects, within the range of 4 to20, preferably 4.5 to 10.

In one preferred aspect, the natural rubber has a weight-averagemolecular weight of at least 0.5×10⁶ g/mol and a polydispersity of atleast 4.5.

An advantage of the process is the ability to use high-molecular-weightnatural rubber (commonly available as “technically specified rubber” or“TSR”), which is less expensive than more highly processed rubbers suchas the “constant viscosity” or “CV” grades of rubber (e.g., CV60 rubber,which has a Mooney viscosity of about 60).

The nature of the rubber can vary considerably depending upon thegeographic source, season, species of tree, processing performed by thesupplier, and other factors. Suitable natural rubber includes “SIR-3”rubber, a TSR grade of standard Indonesian rubber. Suitable naturalrubber also includes SMR, STR, and SVR grades of rubber from,respectively, Malaysia, Thailand, and Vietnam. Suitable natural rubbercan also originate from China, India, the Philippines, the Ivory Coast,and other nations.

The rubber typically makes up 10 to 50 wt. %, preferably 20 to 40 wt. %or 25 to 35 wt. % of the pressure-sensitive adhesive mixture, which willinclude the C₁₂-C₂₄ fatty acid, tackifiers, fillers, and other optionalcomponents such as antioxidants, homogenizers, or flow aids.

Tackifier

The pressure-sensitive adhesives include one or more tackifiers.Suitable tackifiers are well known in the art, and many are commerciallyavailable from Eastman, Arizona Chemical, Cray Valley, Struktol, andother suppliers. Suitable tackifiers include, for example, rosin acids,partially polymerized rosin acids (e.g., rosin acid dimers), rosinesters, terpene ester resins, hydrocarbon resins, terpene resins,terpene-phenol resins, terpene-hydrocarbon resins, and the like, andmixtures thereof. Piccotac™ 1098 aliphatic C₅ hydrocarbon resin(Eastman) and its combination with a rosin acid dimer provide excellenttackification for making pressure-sensitive adhesives suitable for ducttape applications.

The amount of tackifier used in formulating the pressure-sensitiveadhesive will depend on many factors, including the source, nature,viscosity, and molecular weight of the natural rubber, the nature of thetackifier, the desired adhesive properties, the identity and amounts ofother adhesive components, extruder processing conditions, and otherfactors. Generally, the amount of tackifier used will be within therange of 10 to 50 wt. %, preferably 20 to 40 wt. % or 25 to 35 wt. % ofthe pressure-sensitive adhesive mixture.

Most of the tackifier (at least 80%, preferably at least 85%) ispreferably introduced into a section of the extruder that is subsequentto the first section, i.e., downstream from where the natural rubber isintroduced. Some tackifiers can act as lubricants that inhibit orprevent proper mastication of the rubber, especially if too muchtackifier is introduced too early in the process.

Filler

The pressure-sensitive adhesive will include one or more fillers.Fillers normally do not act as accelerators, catalysts, vulcanizationaids, or the like. Instead, they provide volume, thickening, and color(frequently white, yellow, or tan) to the pressure-sensitive adhesive.Suitable fillers are well known in the adhesive field. Examples includecalcium carbonate, magnesium carbonate, magnesium silicate, titaniumdioxide, clays, dolomites, silicas, aluminas, and the like, and mixturesthereof. Calcium carbonate and titanium dioxide are preferred.

The amount of filler needed for the pressure-sensitive adhesive variesand depends on many of the same factors described above for thetackifiers. Additionally, the flow properties and desired viscosity ofthe adhesive will be factors to consider. Generally, the total amount offiller used will be within the range of 20 to 60 wt. %, 25 to 50 wt. %,or 30 to 40 wt. %, based on the amount of pressure-sensitive adhesive.

The C₁₂-C₂₄ Fatty Acid

Pressure-sensitive adhesives made by the inventive process include 0.1to 5 wt. %, 0.15 to 3 wt. %, or 0.2 to 2 wt. %, based on the amount ofmixture, of an added C₁₂-C₂₄ fatty acid. Natural rubber already containscertain fatty acids in various proportions depending upon the source ofthe rubber. The amount discussed here is in addition to any amount thatoccurs naturally in the rubber. Surprisingly, we found that addition ofminor proportion of a C₁₂-C₂₄ fatty acid allows high-molecular-weightnatural rubber to be converted to a pressure-sensitive adhesive using asingle- or twin-screw extruder, mild conditions, and reasonably shortresidence times.

Suitable C₁₂-C₂₄ fatty acids can be saturated, monounsaturated, orpolyunsaturated. Examples include lauric acid, myristic acid, palmiticacid, stearic acid, arachidic acid, behenic acid, palmitoleic acid,oleic acid, linoleic acid, and the like, and mixtures thereof. SaturatedC₁₄-C₂₀ fatty acids, especially saturated C₁₆-C₁₈ fatty acids arepreferred. Preferred fatty acids have melting points greater than 25°C., preferably at least 35° C., at least 50° C., or within the range of60° C. to 75° C. In a preferred aspect, the fatty acid is stearic acid,palmitic acid, or a mixture thereof. Stearic acid is particularlypreferred.

Process for making the PSA

In a first process step for making the pressure-sensitive adhesive, amixture comprising the natural rubber, tackifier, filler, and C₁₂-C₂₄fatty acid is introduced into a first section of the single- ortwin-screw extruder and is masticated. “First section” as used hereinrefers to one or more extruder sections in which the formulationcomponents are first introduced and are subsequently mixed andmasticated. In practice there may be two or more discrete extrudersections in which these steps are accomplished. For instance, in FIG. 1,“first section” refers to S1 and S2. The temperature in this firstsection is mild, i.e., within the range of 10° C. to 45° C., preferably20° C. to 30° C. Residence time in the first section is typically lessthan two minutes, preferably less than one minute.

In a second process step, mastication of the mixture continues in atleast one subsequent extruder section at a temperature within the rangeof 45° C. to 100° C., preferably 50° C. to 90° C., in the presence ofadditional tackifier. The additional tackifier can be introduced in oneor more of the subsequent extruder sections. For instance, in FIG. 1,additional tackifier is introduced at S3 and S5 using hoppers H2 and H3,respectively.

The total residence time of the natural rubber in the extruder ispreferably less than five minutes, more preferably less than threeminutes.

An advantage of the process is the ability to achieve desirablemastication of the natural rubber in the absence of a chemical peptizer.Peptizers have long been used to accelerate degradation of naturalrubber to make it easier to process. However, peptizers are preferablyavoided because they can negatively impact performance of thepressure-sensitive adhesive.

PSA Product

Pressure-sensitive adhesives produced in accord with the inventiveprocess have good to excellent homogeneity. In contrast, when adhesivesare produced under similar conditions without the added C₁₂-C₂₄ fattyacid, the homogeneity of the adhesive can suffer, even resemblingcottage cheese (see Comparative Example 2 in Table 1, below).

The pressure-sensitive adhesives produced according to the inventiveprocess have relatively low viscosities compared with similar adhesivesproduced in the absence of the added C₁₂-C₂₄ fatty acid. In particular,the adhesives have a viscosity, measured at 150° C. and a shear rate of5000 s⁻¹, at least 15% lower, preferably at least 20 or 30% lower, thanthe viscosity of a pressure-sensitive adhesive prepared by a similarprocess without the added C₁₂-C₂₄ fatty acid. Compare the viscosities ofthe adhesives of Control 1 and Example 1 in Table 1, below.

Duct Tape Production

Pressure-sensitive adhesives made by the inventive process arewell-suited for the manufacture of a variety of tapes, particularly ducttapes. Duct tapes are constructed with a flexible polymer backing, acloth scrim, and the pressure-sensitive adhesive. Typical duct tapeconstruction is shown in U.S. Publ. No. 2003/0215628 and U.S. Pat. Nos.4,303,724, 5,108,815, and 5,271,999, the teachings of which areincorporated herein by reference. Reduced-viscosity adhesives thatretain the good adhesion and flexibility of high-molecular-weightnatural rubbers are valuable because they can more easily penetrate theopenings in the cloth scrim and form a good bond with both the backingand the scrim.

The backing is normally a flexible polymer, often a polyolefin such aslow-density polyethylene (LDPE). The thickness of the backing can vary,but it typically falls within the range of 0.5 to 5 mils, preferably 1to 4, or 2 to 3 mils. Depending on the intended use, the backing may bea single layer or multiple layers. U.S. Publ. No. 2003/0215628, forinstance, illustrates a multi-layer backing with an LDPE outer layer, acore of LDPE, HDPE, of a blend thereof, and an inner LDPE layer. Thebacking may include, particularly in any outer layer, light-stabilizers,pigments, gloss aids, adhesion promoters (e.g., acrylate homopolymersand copolymers), or other additives. In a preferred aspect, the backingcomprises low-density polyethylene.

The cloth scrim consists of fibers arranged perpendicularly to form amesh pattern. The fibers can be natural or synthetic. Natural fibersinclude, for instance cotton, flax, hemp, wool, and the like,particularly cotton. Synthetic fibers include polyesters, rayon,acetate, nylon, and the like, especially polyester. In some aspects, thecloth scrim may be produced from combinations or blends of natural andsynthetic fibers.

Cloth scrims are identified by the number of threads in the machinedirection by the number of threads in the cross direction that will fitwithin a 1″ square of fabric. Thus, a thread count of “18×6” means thatthe cloth has 18 counts in the machine direction and 6 in the crossdirection in the 1″ square. The thread count needed for a particularduct tape will depend on its intended use (e.g., light duty versus heavyduty). A denser scrim might have up to 40 or 50 machine-directionthreads and up to 20 or 30 cross-direction threads. One premium cottonscrim, for instance, has a thread count of 44×28.

The thickness of the finished duct tape can vary depending on theintended use. Thicknesses for the finished tape will range from 2 to 20mils, preferably 5 to 15 mils or 7 to 10 mils.

Duct tapes can be manufactured by any suitable method. For instance, thepressure-sensitive adhesive and backing can be co-extruded along withthe cloth scrim. In another approach, a calendering process, asdescribed below, is used.

Thus, in one suitable approach to making duct tapes, thepressure-sensitive adhesive leaves the twin-screw extruder and istransferred to a roller mill that consists of closely spacedstainless-steel rollers. The rollers are hollow to allow them to beheated with water or other process fluids. The rollers are attached tohigh torque gears and a motor that can rotate them at a desired speed.The cylinders are fixed in place so that only a small gap exists betweenthem. As the rollers turn, the adhesive being fed forms a thin sheetacross their surface.

Cloth scrim backed with LDPE or other flexible polymers can be fed froma storage roll using another set of rollers to coat the backed scrimwith adhesive. Operators can control the gaps between the rollers tocontrol the amount of adhesive being applied to the cloth scrim in thiscalendering process.

Thereafter, the tape fabric is wound onto large cardboard cores (e.g.,about 5′ wide, 3′ diameter). When enough tape has been coated and theroll is full, it is removed from its spindle and moved to another areawhere it can be cut to the proper size. This is done by unspooling thelarge rolls onto a machine equipped with a series of knives. The knivescut the product into narrow segments that can be rewound onto smallercardboard cores. Finally, the rolls are packaged for sale.

Duct Tape Properties:

Duct tapes made by the inventive process are generally characterized byone or more improved properties, including, for example, adhesion tosteel, adhesion to the backing, adhesion bond strength, tack, seven-dayclean removability, and other properties. Many of the test methods usedto evaluate these properties are explained in more detail below.

As shown in Tables 2-4 below, duct tapes made in accord with theinventive process can provide a 20% or more improvement in adhesion tosteel and a 25% or more improvement in adhesion to the backing.Moreover, the tapes demonstrate a substantial improvement over thecontrols in seven-day clean removability from difficult substrates,particularly ceramic tile and marble.

The following examples merely illustrate the invention; the skilledperson will recognize many variations that are within the spirit of theinvention and scope of the claims.

Continuous Process for Manufacture of a Pressure-Sensitive Adhesive

A co-rotating twin-screw extruder (F. Ili Maris, S.p.A., Italy)configured as in FIG. 1 and having a screw diameter of 150 mm and alength-to-diameter ratio of 54 is used.

Sections 2 and 4 (S2 and S4) are rubber mastication sections in whichthe molecular weight of the rubber is reduced by mechanical energy.Sections 1, 3, and 5 (S1, S3, and S5) are conveying sections in whichmaterials are transported to the next extruder section. Section 6 (S6)is both a dispersion and a conveying section, where the final adhesiveis discharged to a downstream conveying belt before being charged to acalender for duct tape processing. The temperatures used in each sectionare as follows: S1: 70° F.; S2: 70° F.; S3: 100° F.; S4: 115° F.; S5:115° F.; and S6: 170° F.

Hoppers 1-3 (H1-H3) are configured as shown in FIG. 1. The followingadhesive components in the amounts shown in Table 1 are introduced intoHopper 1: natural rubber (SIR-3 rubber, a technically specified rubber(TSR) grade from Indonesia), calcium carbonate, low-densitypolyethylene, Piccotac™ 1098 hydrocarbon resin (C₅ aliphatic resin,product of Eastman), Resin P rosin acid dimer (product of Eastman),titanium dioxide, Wingstay® L polyphenolic antioxidant (butylatedreaction product of p-cresol and dicyclopentadiene, product of Omnova),and Industrene® R stearic acid/palmitic acid mixture (product of PMCBiogenics).

The SIR-3 natural rubber feed has a Mooney viscosity of 94 (measuredwith top and bottom plates set to 212° F.; 1″ cube sample) and thefollowing molecular weight characteristics (by gel-permeationchromatography): M_(n): 205,000; M_(w): 2,174,000; polydispersity index:10.6. GPC conditions: Waters Breeze™ 2 system; Styragel® HR1 andStyragel® E1 columns (Waters); 30-mg sample in 10 mL of tetrahydrofuran.

These components are introduced into Hopper 2: calcium carbonate,Piccotac™ 1098 hydrocarbon resin, and AO2246 antioxidant(2,2′-methylene-bis(4-methyl-6-tert-butylphenol).

For Hopper 3, the only component added is Piccotac™ 1098 hydrocarbonresin.

TABLE 1 Natural Rubber Formulation, Point of Extruder Entry, and ResultsControl 1 Comp. Ex Comp. Ex. Ex. 1, Example wt. % 1, wt. % 2, wt. % wt.% Hopper 1 SIR-3 natural rubber 29.9 29.9 29.9 29.7 CaCO₃ 20.7 20.7 20.720.7 LDPE 5.0 5.0 5.0 4.7 Piccotac ™ 1098 resin 3.6 2.6 2.6 2.6 Resin Prosin acid resin 2.6 2.6 2.6 2.6 TiO₂ 0.89 0.89 0.89 0.89 Wingstay ® L0.34 0.34 0.34 0.34 antioxidant Industrene ® R 0 0 0 0.5 stearic acidHopper 2 CaCO₃ 13.4 13.4 13.4 13.4 Piccotac ™ 1098 resin 6.81 7.31 5.815.81 AO 2246 antioxidant 0.82 0.82 0.82 0.82 Hopper 3 Piccotac ™ 1098resin 15.94 16.44 17.94 17.94 Total wt. % 100 100 100 100 Adhesivehomogeneity excellent marginal poor excellent Adhesive viscosity 28.527.5 — 22.5 (Pa · s) at 300° F. and shear rate 5000s⁻¹

As shown in Table 1, the main difference between Control 1 andComparative Example 1 is the distribution of the Piccotac™ 1098 resinamong Hoppers 1-3. The total amount of Piccotac™ 1098 resin is the samein these examples. However, in Comparative Example 1, Hopper 1 has lessPiccotac™ 1098 than the amount used in Control 1, while both Hoppers 2and 3 have more Piccotac™ 1098 than the amount used in the control.Although there is a slight reduction in viscosity for ComparativeExample 1, the resulting adhesive has only marginal homogeneity.

Comparative Examples 1 and 2 differ in the distribution of Piccotac™1098 between Hoppers 2 and 3. In Comparative Example 2, Hopper 2 hasless Piccotac™ 1098 than the amount used in Comparative Example 1, whileHopper 3 has more Piccotac™ 1098 than the amount used in ComparativeExample 1. However, in Comparative Example 2, the adhesive appearance ispoor (like cottage cheese), and the viscosity is not measured.

Example 1 includes 0.5 wt % of stearic acid/palmitic acid mixture inHopper 1 with a slight reduction in the amounts of natural rubber andLDPE to maintain a constant total mass balance. Inclusion of the fattyacid is the main difference between Example 1 and Comparative Example 2.The distribution of Piccotac™ 1098 among Hoppers 1-3 is the same forExample 1 and Comparative Example 2. Interestingly, the resultingadhesive in Example 1 is homogeneous and has a viscosity (22.5 Pa·s)that is 21% lower than that of the Control.

Thus, adding 0.5 wt. % of C₁₆-C₁₈ fatty acid in Hopper 1 improves themixing uniformity of the adhesive and also decreases adhesive viscosity.Without any fatty acid in Hopper 1, the operator has to reduce theoutput of extruder or tolerate the high adhesive viscosity due toinadequate mastication of the natural rubber.

Test Methods for Adhesive Tapes

Generally, the test methods and equipment used are described in PressureSensitive Tape Council (PSTC) Test Methods, 15th Ed. Unless otherwiseindicated, room conditions for all tests are kept at 73.4±3.6° F. and50±2% relative humidity. Similar test methods are also described in U.S.Publ. No. 2003/0215628, the teachings of which are incorporated hereinby reference.

Adhesion to Steel (PSTC 101/Test Method A)

Before use, each test panel is cleaned. Acetone or methyl ethyl ketoneis used for metals. Isopropyl alcohol is used for plastics. Delicatesurfaces are simply wiped to remove dust. A 1″-wide strip of tape isapplied to a standard test panel with controlled pressure. Pressure isapplied using either a mechanically operated 4½ lb. roller or ahand-operated, PSTC-approved 4.5 lb. rubber-covered roller. The tape ispeeled from the panel at a 180° angle at a rate of 12±0.05 inches perminute, during which time the force required to peel is measured.

Adhesion to Backing (PSTC 101/Test Method B)

A strip of tape is applied to a rigid panel. A second strip of the tapeis applied to the backing of the first strip and tested for peeladhesion as described in Method A.

Shear Adhesion (PSTC 107/Procedure A)

A 1″-wide strip of tape is applied to a standard steel panel undercontrolled roll down. Pressure is applied using either a mechanicallyoperated 4½ lb. roller or a hand-operated, PSTC-approved 4.5 lb.rubber-covered roller. The panel is mounted vertically, a standard massof 1 kg is attached to the free end of the tape, and the time to failureis determined.

Unwind Adhesion (PSTC 8)

Unwind adhesion is the force required to remove the tape from the rollunder prescribed conditions.

The mandrel is fitted into the lower jaw of the tensile tester, and thefree end of the mandrel is inserted through the roll of tape. Thelocation of the tape roll is adjusted so that when the free end of thetape is placed in the upper clamp, the unwound tape remains in avertical plane between the two clamps. Clamp separation is operated at12 inches per minute. After 1 inch of tape has been unwound, the averagevalue of the unwind force is observed while unwinding the next 3 inches.

High-Speed Unwind Adhesion (PSTC 13)

High-speed unwind adhesion is the force required to remove the tape fromthe roll under prescribed conditions. An unwind machine capable ofunwinding the roll at a rate of 200 ft/min and having means of sensingand indicating the unwind force measured parallel to the unwinding stripis used. The test is performed at an unwind rate of 100 ft/min, and theunwind adhesion force is measured.

Rolling Ball Tack (PSTC 6)

The rolling ball tack test is one measure of the capacity of theadhesive to form a bond with the surface of another material upon briefcontact under virtually no pressure. Prior to each roll, an 11-mmstainless-steel ball is carefully cleaned with methyl ethyl ketone. Theball is wiped with a paper towel to remove any remaining residue. Theball is released from the top of a ramp and is allowed to roll to a stopon the adhesive. The distance from where the ball stops to the pointwhere the ball first touches the adhesive surface is measured andrecorded.

Adhesion Bond Strength (Modified PSTC 8)

Adhesion bond strength is a measure of the force required to separatethe fabric from the polyolefin backing in duct tape. It should begreater than the force required to unwind the roll of tape to avoid thepossibility of delamination of the tape during unwinding.

1. A roll of tape is prepared by unwinding a few inches of tape from theroll. Then, using a sharp knife, and holding both the roll and the looseend of the tape tightly, a cut is made across the entire top layer ofpolyethylene remaining on the roll as close as possible to the exposedadhesive surface of the unwound portion.

2. After cutting the top layer of polyethylene, an attempt is made toseparate the polyethylene from the fabric to which it is bonded by firstpulling the polyethylene layer loose by hand. Then, if the polyethylenewill release from the fabric, the loose end of the tape is pulled.

3. If the polyethylene will separate from the fabric, the adhesion bondstrength is measured.

4. The mandrel is fitted into the lower jaw of the tensile tester, andthe free end of the mandrel is inserted through the roll of tape. Thelocation of the tape roll is adjusted so that when the free end of thetape is placed in the upper clamp, the unwound tape remains in avertical plane between the two clamps. Clamp separation is operated at12 inches per minute.

5. After 1 inch of tape has been unwound, the average value of theunwind force is observed while unwinding the next 3 inches. The adhesionbond strength is reported in pounds per inch, to the nearest 0.1 lb. Ifthe tape width is not 1 inch, the test result is divided by the actualwidth of the tape.

Delta Bond

Delta bond is calculated by subtracting adhesion bond strength (lb/in)from unwind adhesion (lb/in).

Accelerated Aging (PSTC 9)

This procedure simulates the natural aging of the tape from 9 to 15months. The tests performed are an estimate of how well the tape willreact over time. Sample rolls are conditioned at 120° F., 80% relativehumidity, for 36 hours prior to testing using the above-describedprocedures.

Preparation of Duct Tapes using the Pressure-Sensitive Adhesive

Duct tapes are made by a calender process as described earlier. Adhesivemade from SIR-3 natural rubber (Mooney viscosity 93) is laminated onto apolyethylene backing (about 2.5 mils) along with a cloth scrim (threadcount 18×6) according to the desired product specifications. The targettape thickness is 9.3 mils. Tape physical properties from the adhesivesprepared above as Control 1 and Example 1 are shown in Table 2.

TABLE 2 Properties of Duct Tapes Control 1 Ex. 1 Properties of unagedtapes Adhesion to steel (oz/in)  53.1  61.8 Adhesion to backing (oz/in) 23.6  29.5 Shear (steel, min) 5700+ 5700+ Bond (lb/in)   4.43   4.90Unwind (lb/in)   1.30   1.23 Delta bond (lb/in)   3.13   3.67 High-speedunwind (oz/in)  11.1  10.9 Rolling ball (in)   1.48   1.66 Adhesiveweight (oz/yd²)   4.40   4.79 Tape gauge (mils)   8.88   9.07 Propertiesof aged tapes (36 h, 120° C., 80% RH) Aged Adhesion to steel (oz/in) 60.0  58.5 Aged Adhesion to backing (oz/in)  26.6  31.3 Aged Bond(lb/in)   6.02   7.02 Aged Unwind (lb/in)   2.31   2.53 Delta bond(lb/in)   3.71   4.49 Aged High-speed unwind (oz/in)  52.9  54.9 AgedRolling ball (in)   1.01   1.90

As shown in Table 2, adhesion to steel increases 16% (from 53 to 62oz/in) when a C₁₆-C₁₈ fatty acid is included in Hopper 1. The adhesionbond strength also increases from 4.4 to 4.9 lb/in, which suggestsimproved resistance to delamination and defect formation during use ofthe tape.

Residue Testing:

Freshly produced tapes are applied to various substrate surfaces toevaluate their performance in resisting adhesive residue for a dwelltime of 7 days. The tapes are removed at peel rate of 3 in./s and thesurfaces are examined for the presence of any adhesive residue.

The duct tape from Control 1 leaves a bad residue on marble, ceramictile, and laminate flooring, but it leaves no residue on carpet.

In contrast, the duct tape from Example 1 leaves no residue on marble,ceramic tile, laminate flooring, or carpeting. Thus, the inventiveadhesive demonstrates a substantial improvement over the control inseven-day clean removability from difficult substrates, particularlyceramic tile and marble.

Additional duct tapes are made by a calender process. Adhesive made fromSIR-3 natural rubber (Mooney viscosity 91) is laminated onto apolyethylene backing (about 2.5 mils) along with a cloth scrim (threadcount 18×6) according to the desired product specifications. Target tapethickness: 9.3 mils. Tape physical properties from the adhesivesprepared above as a control and this Example 2 are shown in Table 3.

TABLE 3 Properties of Duct Tapes Control 2 Ex. 2 Properties of unagedtapes Adhesion to steel (oz/in)  58.7  62.2 Adhesion to backing (oz/in) 27.7  25.7 Shear (steel, min) 5700+ 5700+ Bond (lb/in)   4.64   5.06Unwind (lb/in)   1.16   1.20 Delta bond (lb/in)   3.58   3.86 High-speedunwind (oz/in)   6.38  11.5 Rolling ball (in)   3.27   1.59 Adhesiveweight (oz/yd²)   5.36   5.49 Tape gauge (mils)   9.75   9.52 Propertiesof aged tapes (36 h, 120° C., 80% RH) Aged Adhesion to steel (oz/in) 61.1  48.0 Aged Adhesion to backing (oz/in)  31.2  25.0 Aged Bond(lb/in)   7.97   7.13 Aged Unwind (lb/in)   5.23   4.37 Delta bond(lb/in)   5.23   4.37 Aged High-speed unwind (oz/in)  33.8  58.9 AgedRolling ball (in)   2.10   0.87

As shown in Table 3, adhesion to steel increases (from 59 to 62 oz/in)when a C₁₆-C₁₈ fatty acid is included in Hopper 1. The adhesion bondstrength also increases from 4.6 to 5.1 lb/in, which suggests improvedresistance to delamination and defect formation during use of the tape.

Residue Testing:

The duct tape from Control 2 leaves a bad residue on marble, ceramictile, and laminate flooring, but it leaves no residue on carpet.

In contrast, the duct tape from Example 2 leaves no residue on marble,ceramic tile, laminate flooring, or carpeting. Thus, the inventiveadhesive again demonstrates a substantial improvement over the controlin seven-day clean removability from difficult substrates, particularlyceramic tile and marble.

Additional duct tapes are made by a calender process. Adhesive made fromSIR-3 natural rubber (Mooney viscosity 95) is laminated onto apolyethylene backing (about 2.25 mils) with a thin layer of ethylenemethyl acrylate copolymer (EMA) as an adhesion promoter, along with acloth scrim (thread count 38×14) according to the desired productspecifications. The target tape thickness is 12.5 mils. Tape physicalproperties from the adhesives prepared above as a control and thisExample 3 are shown in Table 4.

For this trial, a third lot of SIR-3 rubber is used. A denser scrim(38×14) is used. Target tape thickness: 12.5 mils. Adhesion to steelincreases 19% from 50.4 to 59.9 oz/in for Example 3 as compared toControl 3. Adhesion bond to the backing is not tested because thebacking includes a layer of EMA adhesion promoter, which should provideadequate adhesion bond. The tape of Example 3 has a lower rolling ballnumber compared with that of the control, indicating higher tack.

TABLE 4 Properties of Duct Tapes Control 3 Ex. 3 Properties of unagedtapes Adhesion to steel (oz/in)  50.4  59.9 Adhesion to backing (oz/in) 23.6  31.0 Shear (steel, min) 5700+ 5700+ Unwind (lb/in)   1.25   1.17High-speed unwind (oz/in)  17.7  18.9 Rolling ball (in)   1.30   0.82Adhesive weight (oz/yd²)   9.00   9.54 Tape gauge (mils)  12.0  12.4Backing gauge (mils)   2.30   2.30 Properties of aged tapes (36 h,    120° C., 80% RH)     Aged adhesion to steel (oz/in)  52.4  42.5 Agedadhesion to backing (oz/in)  26.0  23.8 Aged unwind (lb/in)   2.06  1.89 Aged high-speed unwind (oz/in)  48.8  49.0 Aged rolling ball (in)  0.77   1.42

The preceding examples are meant only as illustrations; the followingclaims define the inventive subject matter.

I claim:
 1. A continuous process for making a pressure-sensitiveadhesive, comprising: (a) in a single- or twin-screw extruder, in afirst section at a temperature within the range of 10° C. to 45° C.,masticating a mixture comprising natural rubber having a Mooneyviscosity within the range of 85 to 100, a tackifier, and a filler inthe presence of 0.1 to 5 wt. %, based on the amount of mixture, of anadded C₁₂-C₂₄ fatty acid; and (b) continuing to masticate the mixture inat least one subsequent extruder section at a temperature within therange of 45° C. to 100° C. in the presence of additional tackifier toproduce a homogeneous pressure-sensitive adhesive having a viscosity,measured at 150° C. and a shear rate of 5000 s⁻¹, at least 15% lowerthan the viscosity of a pressure-sensitive adhesive prepared by asimilar process without the added C₁₂-C₂₄ fatty acid.
 2. The process ofclaim 1 wherein the natural rubber has a Mooney viscosity within therange of 90 to
 98. 3. The process of claim 1 wherein the natural rubberhas, as measured by gel permeation chromatography, a weight-averagemolecular weight of at least 0.5×10⁶ g/mol and a polydispersity of atleast 4.5.
 4. The process of claim 1 wherein the tackifier comprises aresin selected from the group consisting of rosin acids, partiallypolymerized rosin acids, rosin esters, terpene ester resins, hydrocarbonresins, terpene resins, terpene-phenol resins, terpene-hydrocarbonresins, and mixtures thereof.
 5. The process of claim 1 wherein thefiller comprises a compound selected from the group consisting ofcalcium carbonate, magnesium carbonate, magnesium silicate, titaniumdioxide, clays, dolomites, silicas, aluminas, and mixtures thereof. 6.The process of claim 1 wherein the fatty acid has a melting pointgreater than 25° C.
 7. The process of claim 1 wherein the fatty acid isselected from the group consisting of stearic acid, palmitic acid, andmixtures thereof.
 8. The process of claim 1 performed in the presence of0.2 to 2 wt. %, based on the amount of mixture, of the added C₁₂-C₂₄fatty acid.
 9. The process of claim 1 wherein the pressure-sensitiveadhesive has a viscosity, measured at 150° C. and a shear rate of 5000s⁻¹, at least 20% lower than the viscosity of a pressure-sensitiveadhesive prepared by a similar process without the added C₁₂-C₂₄ fattyacid.
 10. The process of claim 1 wherein the temperature in the firstsection is within the range of 20° C. to 30° C.
 11. The process of claim1 wherein at least 80 wt. % of the tackifier is introduced in a sectionof the extruder subsequent to the first section.
 12. The process ofclaim 1 performed in the absence of a chemical peptizer.
 13. The processof claim 1 wherein the residence time of the natural rubber in theextruder is less than five minutes.
 14. A pressure-sensitive adhesivemade by the process of claim
 1. 15. A duct tape comprising the adhesiveof claim 14 and a backing which comprises a polyolefin and a clothscrim.
 16. The duct tape of claim 15 having improved adhesion to steelcompared with a similar duct tape made with a pressure-sensitiveadhesive produced without the added fatty acid.
 17. The duct tape ofclaim 15 having improved adhesion bond strength to the polyolefinbacking compared with a similar duct tape made with a pressure-sensitiveadhesive produced without the added fatty acid.
 18. The duct tape ofclaim 15 having improved seven-day clean removability on at least one ofmarble, ceramic tile, or laminate flooring when compared with a similarduct tape made with a pressure-sensitive adhesive produced without theadded fatty acid.
 19. A process for making a duct tape, comprising: (a)in a single- or twin-screw extruder, in a first section at a temperaturewithin the range of 10° C. to 45° C., masticating a mixture comprisingnatural rubber having a Mooney viscosity within the range of 85 to 100,a tackifier, and a filler in the presence of 0.1 to 5 wt. %, based onthe amount of mixture, of an added C₁₂-C₂₄ fatty acid; (b) continuing tomasticate the mixture in at least one subsequent extruder section at atemperature within the range of 45° C. to 100° C. in the presence ofadditional tackifier to produce a homogeneous pressure-sensitiveadhesive having a viscosity, measured at 150° C. and a shear rate of5000 s⁻¹, at least 15% lower than the viscosity of a pressure-sensitiveadhesive prepared by a similar process without the added C₁₂-C₂₄ fattyacid; and (c) calendering the pressure-sensitive adhesive onto a backingcomprising a polyolefin and a cloth scrim to produce the duct tape. 20.The process of claim 19 wherein the polyolefin is low-densitypolyethylene.